What makes a cool roof “cool”?

Jasmine O'Donoghue

A whole range of design elements fall under the umbrella term of “cool roofs”. Essentially, a cool roof is one that reflects the suns heat and emits absorbed radiation back into the atmosphere at a higher rate than standard materials, literally staying cooler than a standard roof. This reduces the amount of heat transferred into the building below, keeping it at a more constant temperature.[1]

Cool roofs are one element of passive design which can help keep a building and its occupants comfortable by lessening temperature fluctuations. Incorporating a cool roof into a design in the appropriate climate can make a building “greener” by reducing cooling energy costs, minimise the use or need for installation of air conditioning systems and subsequently reduce utility bills. Cool roofs can also mitigate a community’s Urban Heat Island Effect.

UNDERSTANDING COOL ROOFS

A cool roof can be installed on any building. Some elements are built into materials before construction, and some are applied after, allowing a standard roof to be post-fitted with cool roof technology. Without a cool roof, on a 35°C day, the roof space can become superheated up to 90°C and the temperature of the rooms below can become unbearable, even with insulation.[2] These temperature extremes can have an impact on the durability and the lifespan of the whole roof structure.

There are a few terms which come into play when determining the “coolness” of a roof. The ability of a material to reflect the solar radiation of light, infrared and UV (Solar Reflectance) and the measure of the material’s ability to absorb and re-radiate heat into the atmosphere (Thermal Emittance) is combined into what is known as the Solar Reflectance Index (SRI).[3] SRI is a measure of the constructed surface's ability to reflect solar heat.[4] The higher the SRI value, the lower its surface temperature and consequently, the heat gain into the building.

THE COLOUR DEBATE

Five of the colours in the Colorbond steel range qualify as solar reflective (light coloured) roofing under the Green Star Communities tool. Image: Colorbond

The role of colour in a cool roof’s ability to reflect sun comes down to basic physics. Darker coloured roofs made from standard materials will absorb more heat than lighter coloured standard roofs as black materials absorb all wavelengths of light and reflect none while white materials reflect all wavelengths of light and therefore, absorb the least heat.

This was demonstrated by a study at James Cook University, which used building simulation to compare the heat gain from light and dark-coloured roof surfaces. The research found for north Australia, a light-coloured roof has about 30 per cent lower total (air temperature difference and solar-driven) heat gain than a dark-coloured one.[5]

A US study by the Lawrence Berkeley National Laboratory went a step further and tested black, white and green (vegetated) roof types to determine the SRI and which was the most cost effective. A roof with a clean, smooth bright white surface was found to have an SRI of 107, the surface of a standard grey roof had an SRI of just 19 and a roof with a clean, smooth “cool colour” surface, such as a cool red tile, had an SRI of 38.[6] White roofs also came out on top for cost and were found to be the most cost-effective over a 50-year time span in the United States. The paper “strongly recommended building code policies that phase out dark-coloured roofs in warm climates to protect against their adverse public health externalities”.[7]

While the numbers may vary between climates, the research suggests a lighter coloured roof will absorb less heat than a darker coloured one. So why aren’t all Australian roofs a lighter colour?

AUSTRALIAN TRENDS

Bathurst Sustainable Living House, a winning entry in an invited competition for a demonstration house to be built by Bathurst Regional Council, uses a light coloured roof. Image and design by Envirotecture

Trends in the Australian roof domain are dependent on location. Designing for climate is of utmost importance when creating a building where occupants remain thermally comfortable with minimal heating or cooling. Your Home recommends using light coloured roof materials in eight out of nine (excluding Alpine) climate zones within Australia. But depending on the geographical location, this is not always well practised.

Dick Clarke Director, Envirotecture said in the Top end, pretty much all roofs are white but in southern states, there has been “a misguided tendency for local government to mandate darker colours as a means of getting buildings to blend in to the natural environment”.

“I say misguided, because we should not pretend that we are not building things - we can make buildings which are clearly there, but are harmonious without being exactly the same as the tree next to it,” he explained.

Clarke said there is also a misguided belief that light colours create glare for neighbours, but “it is texture that creates glare by means of low angle reflectivity.

“I have seen massive glare coming off a neighbour’s new black tiled roof, while an older white metal roof has created none,” he said.

For most parts of southern Australia, Clarke explained an enclosed roof space works well, “as long as there is a complex of multi-layered insulation” which uses foil and some bulk insulation in a complementary way.

THE OTHER OPTIONS

Cliff Face House by Fergus Scott Architects and Peter Stutchbury Architecture uses a parasol roof to reportedly great effect. Photography by Michael Nicholson

In addition to colour, another option is a parasol roof, which is entirely open on all sides and the ridge too, preventing hot air from gathering in the roof space.

“These can be hard to achieve in many instances, and many people assume that shifting a little bit of air, such as with one two small diameter roof vents, will do the same thing,” Clarke said.

“Sadly, we have a couple of case studies that show it does not: you need to shift a huge volume to get the parasol effect, but a parasol may do more harm in winter than it does good in summer, so it’s not the universal solution either.”

Another alternative is using cool roof coatings, which come in light colours, right through to dark greys and browns. By definition, cool roof coatings (or paint) have significant higher reflectivity and emissivity when compared with normal roof materials and coatings[8], but in the past some suppliers have found themselves in hot water over their claims.

In November, Dulux was hit with a $400,000 fine over the temperature reducing claims it made regarding its roof and wall paint. Dulux promoted its heat-reflective roof paint, Infracool as being able to reduce the interior temperature of the living zones of a house by up to 10 degrees and its Weathershield Heat Reflect wall paint as being able to significantly reduce the interior temperature of a house. The Court found these claims was false or misleading after Dulux admitted that it did not have any reasonable grounds for making those representations.

PUTTING COOL ROOF PAINT TO THE TEST

A study conducted by the University of Melbourne and commissioned by the City of Melbourne tested the use of cool or white roof paints and compared them to normal roof materials and coatings. It found for a typical residential building with insulation in the ceiling, the attic space would be up to 18.5 degrees cooler with the use of cool roof paints. If the insulation is located under the roof, the cool roof paints have only a minor impact on the temperature of the roof space. A typical commercial and industrial building was found to benefit by approximately three per cent in terms of cooling energy reductions, suggesting many older office style buildings would benefit from the use of these products. The study advised buildings with high cooling loads and minimal insulation will received significant benefits with the use of cool roof paint, while buildings without a significant cooling load, and with typical insulation levels (e.g. residential buildings) can benefit from these products, but it will be sensitive to roof pitch, shading and the level of ceiling insulation.

Regardless of which cool roof method used, it is vital to read up on the evidence backing up a material or technique’s claims, and compare the SRI.